Allies of US President Donald Trump have highlighted new claims by a woman who said she was told about sexual-assault allegations against Joe Biden decades ago.

The comments turn the spotlight on the past behaviour of the presumptive Democratic presidential nominee.

Apparent corroboration has surfaced for elements of two accusations made by Mr Biden's former senate aide Tara Reade, one involving harassment and the second a sexual assault.

Mr Biden has not commented on the allegations but his campaign has denied them and pointed to his record on women's rights and promotion of women in his offices.

Lynda LaCasse, who was one of Ms Reade's neighbours in California, where Ms Reade moved after working for Mr Biden, said in an interview with 'Business Insider' that Ms Reade told her in the mid-1990s Mr Biden had put his hand up her skirt and assaulted her.

Lorraine Sanchez, a former colleague of Ms Reade's in the office of a California state senator, also told the news outlet that Ms Reade told her in the mid-1990s that she "had been sexually harassed by her former boss while she was in DC and as a result of her voicing her concerns to her supervisors, she was let go, fired".

Ms Reade made the harassment accusation last year and she recently offered details of what she said was a sexual assault in a hallway somewhere on Capitol Hill.

The allegations have percolated for weeks, a period in which Mr Biden has become the presumptive Democratic presidential nominee.

Three of Ms Reade's supervisors from the time, to whom Ms Reade says she complained, have said they don't remember her or any complaints from her.

Mr Biden's campaign declined to comment on the reports, pointing to previous statements from deputy campaign manager Kate Bedingfield, who said that while women's accounts of wrongdoing should be examined, the one from Ms Reade "absolutely did not happen".

As part of an in-depth examination published two weeks ago, Ms Reade had said that she described the alleged assault soon afterwards to a friend, to her brother and to her mother.

Her friend corroborated Ms Reade's account of their conversation but declined to be named.

Her brother, Collin Moulton, said that she told him in 1993 that Mr Biden had behaved inappropriately by touching her neck and shoulders. He said later that he recalled her telling him that Mr Biden had put his hand "under her clothes".

Mr Biden has participated in several interviews since the assault allegations emerged but has yet to be asked about them. It has, however, been a topic for other top Democrats, including some of his potential running mates.

"I think this case has been investigated," senator Amy Klobuchar said on MSNBC, pointing to her own work to make it easier to bring such cases forward. "I know the vice-president as a major leader on domestic abuse, I worked with him on that."

Mr Trump has been accused by more than a dozen women of sexual assault. He has denied all of the allegations.

His son, Donald Trump Jr, has repeatedly tweeted about the accusations against Mr Biden in recent days.

Ms Reade worked in Mr Biden's Senate office from December 1992 until August 1993, according to employment records. She had initially said last year that Mr Biden had put his hands on her shoulders and neck. She said she complained to senior Biden aides about feeling uncomfortable, but not about sexual assault.